Optical coherence analysis relies on the use of the interference phenomena between a reference wave and an experimental sample wave or between two parts of a sample wave to measure distances and thicknesses, and calculate indices of refraction of a sample. Optical Coherence Tomography (OCT) is one example technology that is used to perform usually high-resolution cross sectional imaging. It is often applied to imaging biological tissue structures, for example, on microscopic scales in real time. Optical waves are reflected from an object or sample and a computer produces images of in depth cross sections of the object by using information on how the waves are changed upon reflection and by scanning optical waves across the sample surface.
The original OCT imaging technique was time-domain OCT (TD-OCT), which used a movable reference mirror in a Michelson interferometer arrangement. Subsequently, Fourier Domain OCT (FD-OCT) techniques have been developed. One example is time-encoded FD-OCT, which uses a wavelength swept source and a single detector; it is referred to as Swept Source OCT (SS-OCT). Another example is spectrum encoded FD-OCT, which uses a broadband source and spectrally resolving detector system.
These various OCT techniques offer different performance characteristics. FD-OCT has advantages over TD-OCT in speed and signal-to-noise ratio (SNR). Of the two FD-OCT techniques, swept-source FD-OCT has distinct advantages over spectrum-encoded FD-OCT because of its capacity for balanced and polarization diversity detection; it has advantages as well for imaging in wavelength regions where inexpensive and fast detector arrays, which are typically required for spectrum encoded FD-OCT, are not available.
Swept source OCT has advantages in some additional respects. The spectral components are not encoded by spatial separation, but they are encoded in time. The spectrum is either filtered or generated in successive frequency steps and reconstructed before Fourier-transformation. Using the frequency scanning swept source, the optical configuration becomes less complex but the critical performance characteristics now reside in the source and especially its tuning speed and accuracy.
The swept sources for swept-source FD-OCT have been typically tunable lasers. The advantages of tunable lasers include high spectral brightness and relatively simple optical designs. The tunable lasers are constructed from a gain medium, such as a semiconductor optical amplifier (SOA), that is located in a resonant optical cavity which also includes a tunable filter such as a rotating grating, grating with a rotating mirror, or a Fabry-Perot tunable filter. Currently, some of the highest speed tunable lasers are based on the laser designs described in U.S. Pat. No. 7,415,049 B1, entitled Laser with Tilted Multi Spatial Mode Resonator Tuning Element, by D. Flanders, M. Kuznetsov and W. Atia. These highly integrated designs allow for a short laser resonant cavity that keeps the round-trip optical travel times within the laser resonant cavity short so that the laser is fundamentally capable of high speed tuning Secondly, the use of micro-electro-mechanical system (MEMS) Fabry-Perot tunable filters combines the capability for wide spectral scan bands with the low mass high mechanical resonant frequency deflectable MEMS mirror membranes, which also have the capacity for high speed tuning.
Another class of swept sources that have the potential to avoid some of the inherent drawbacks of tunable lasers, such as sweep speed limitations, is filtered amplified spontaneous emission (ASE) sources that combine a spectrally broadband light source, typically a source that generates light by ASE, with tunable optical filters and optical amplifiers.
Some of the highest speed devices based on filtered ASE sources are described in U.S. Pat. No. 7,061,618 B2, entitled Integrated Spectroscopy System, by W. Atia, D. Flanders P. Kotidis, and M. Kuznetsov, which describes tunable light sources. A number of variants of the filtered ASE swept source are described, including amplified versions and versions with tracking filters.
Two metrics that characterize the performance of OCT systems hardware are optical interferometer mechanical stability and the electronic bandwidth of the electronic signal processing systems. Many times, the OCT system interferometers are constructed from lengths of optical fiber. Mechanical movement, shock and stress of the optical fiber in these interferometers can affect the propagation of the optical signals in the fiber in terms of optical signal phase and polarization and this can impact system performance of such interferometric optical systems. The sufficiently high electronic bandwidth, also, becomes increasingly important as higher speed, performance and resolution OCT systems are produced. For example, increasing the wavelength tuning speed of the swept source, which produces higher OCT image acquisition speeds, also results in greater requirements for the electronics that are used to sample the resulting optical interference signals.